1. Field of the Invention
The present invention relates to a decentered prism optical system and, more particularly, to a decentered prism optical system of excellent productivity which can be applied to an ocular optical system or an imaging optical system.
2. Discussion of Related Art
Examples of conventionally known decentered prism optical systems include those disclosed in Japanese Patent Application Unexamined Publication (KOKAI) Nos. 7-333551 and 8-234137. The present applicant has also proposed decentered prism optical systems in Japanese Patent Application Unexamined Publication (KOKAI) Nos. 8-320452 and 8-313829. Any of these known decentered prism optical systems uses a rotationally asymmetric surface configuration for a surface having a reflecting action.
In these references, a first surface which is disposed at a side closer to a pupil and which has both transmitting and reflecting actions is formed from a rotationally asymmetric surface, and a second surface which is disposed at a side remote from the pupil and which has only a reflecting action is also formed from a rotationally asymmetric surface, e.g. an anamorphic surface or a toric surface. Therefore, when measurement is performed to see whether or not surface configurations have been finished in conformity to the design values to produce a desired optical system, surface accuracies cannot be measured by an interferometric or other similar method because of rotational asymmetry, and it is necessary to carry out measurement by using a three-dimensional coordinate measuring device. However, a typical three-dimensional coordinate measuring device measures coordinates of points in a point-by-point manner and therefore suffers from the problems that no sufficient measuring accuracy can be attained, and that measurement takes a great deal of time.
Further, the above-described prior art discloses that a third surface having only a transmitting action is designed to be a rotationally symmetric surface. However, the third surface, which has only a transmitting action, has a narrow effective surface area. Therefore, it is difficult to judge from only the third surface whether or not the whole optical system has been produced with the correct configuration. The first surface, which is closer to the pupil and has both reflecting and transmitting actions, has a large effective surface area. Therefore, it is convenient if the first surface is used as a reference for judgment as to whether the whole optical system is distorted or not. In injection molding of a plastic material, it is particularly important to minimize a change in the overall configuration of the optical system, and it is an effective way in mass-production to estimate the overall configuration of the optical system by measuring a surface having a large effective surface (i.e. an area that performs at least one of transmission or reflection of a bundle of light rays in the entire area of the surface).